1. Field of the Invention
This invention concerns an optical system having an optical source that emits a polarized light beam, and which is able to use this polarized light without suffering the effect of the birefringence of the system.
2. Description of the Prior Art
In the prior art, magneto-optic disk devices are typical examples of devices that use polarized light beams.
In optical systems that use magneto-optic disk devices, a signal is read by detecting a slight rotation in the direction of polarization as a change of optical intensity due to the magneto-optic Kerr effect when a laser beam is reflected from the disk surface. The type of laser beam which is used is a linearly polarized beam, for which a change in the direction of polarization is easiest to detect.
In such a system, however, if the lenses or other optical elements used also change the polarization state of the laser beam, the linear polarization of the beam may become elliptical. This weakens the signal and leads to errors when the signal is read.
In general, plastic lenses possess birefringence like crystals, and they change the polarization state of a laser beam. The lenses used in magneto-optic disk devices are therefore glass lenses.
Glass lenses are however heavier than plastic, and it is difficult to finish them aspherically. They therefore have aberrations which can only be corrected by increasing the number of component lenses, leading to an increased load on the actuator.
The lenses used in optical disk devices have to move at high frequencies for focussing and tracking purposes. It is therefore essential to make these devices compact and light weight to lighten the load on the actuator.